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The iPhone 4S includes an infrared LED sensor that remains on whenever its screen is lit up, seemingly to let Siri know when you’re holding the phone up to your face to give it commands, reports iFixit.

All of Apple’s iPhones have included an infrared proximity sensor, but previously it only lit up while you were making a call (or using an app like Skype), which made the phone dim its display and turn off touch functionality. The iPhone 4S, on the other hand, always keeps its IR sensor on when the phone is active.

“Siri is ready and waiting to answer her master’s beck and call at any time,” iFixit writes. “And in order to be as attentive as a personal assistant ought to be, Apple had to design the proximity sensor to be as vigilant as Big Brother, but as cute as Little Sister. So whenever the screen is active, the proximity sensor is active too. Thus, whenever you raise the iPhone 4S to your face, Siri is ready to take orders.”

The always-on IR sensor could be one reason why Apple can’t bring Siri to older iOS devices (though I think that decision has more to do with older iPhones not being fast enough to support Siri). But it’s not like Siri would be unusable without the ever-watching sensor — it’d still be accessible by double-tapping on your home button.

Given that the iPad 2 has the same internal hardware as the iPhone 4S, many are eagerly waiting to see when Apple will port Siri over to its tablet. The lack of an always-on sensor seems like a poor reason to keep Siri off of the iPad 2, especially since it’s not something you’re holding up to your face very often (at least, I hope you’re not).